Assign an appropriate id, 'zero', to any object of class uvc_zero_plotspec.
This is an emergency bug fix to the multiprocessing code.
For unknown reasons, multiprocessing.Semaphore.get_value() started raising
NotImplementedError when a few days ago the whole system was working fine.
The purpose of getting the semaphore value was to tell whether the forked-
off process was still running or not.  So as a quick fix I used a global
variable "running" as a flag - True if the process is running, False if not.
Obviously this won't work if you fork off more than one process.  And it's
ugly.  But it will work until we can figure out what happened with the
multiprocessing.Semaphore class.